Not a sprocket only, but you can buy a 23-tooth freewheel that should fit.
Yes
Yes, they Are Bmx Bikes. I Do Not Recommend Them Thought.
Yes
No. a 9T sprocket will only fit a cassette hub, and not freewheel hubs.
The Mongoose Rebel Is a 20" Freestyle Bmx Bike
It will if you have bmx (or bmx style) cranks.
change out the hub
Yes its a fantastic bike and great for tricks
the easiest way is to go to your local bike shop
Some popular BMX bike brand logos include GT, Haro, Mongoose, and Redline.
If you want a cheap bike mine as well go for a mongoose
You can get a 9 tooth sprocket, you will need a conversion kit to do it. Haro makes a decent one just do a search for Haro 25/9